Phenix v. Prindle

I think, upon an implied contract to pay interest, it cannot be recovered in the action of book debt, though it undoubtedly may by an action on the case; because, in the action on book, the oath of the plaintiff is taken, which ought never to be admitted to prove a fact of that kind. .

Where interest is cbailenged on book, the agreement to allow it ought to be proved by other evidence than the oath of the party; (so ought also any special price or mode of payment agreed on, if he would avail himself of it) for his testimony regularly goes no farther than to the quantity, quality, and delivery of the articles charged; — but if he hath evidence to prove an agreement to allow interest, he may recover it in the same action with the debt, and not be put to a separate suit.